linux-kernel - Re: endianness swapped

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Date:   Sun, 28 Apr 2019 23:31:02 +0200
From:   Angelo Dureghello <angelo@...am.it>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg Ungerer <gerg@...ux-m68k.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux/m68k <linux-m68k@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: endianness swapped

Hi all,

On Sun, Apr 28, 2019 at 08:44:03PM +0200, Arnd Bergmann wrote:
> On Sun, Apr 28, 2019 at 3:59 PM Greg Ungerer <gerg@...ux-m68k.org> wrote:
> > On 28/4/19 7:21 pm, Arnd Bergmann wrote:
> > > On Sun, Apr 28, 2019 at 10:46 AM Geert Uytterhoeven
> > > <geert@...ux-m68k.org> wrote:
> > >> On Sat, Apr 27, 2019 at 10:22 PM Angelo Dureghello <angelo@...am.it> wrote:
> > >>> On Sat, Apr 27, 2019 at 05:32:22PM +0200, Angelo Dureghello wrote:
> > >
> > > Coldfire makes the behavior of readw()/readl() depend on the
> > > MMIO address, presumably since that was the easiest way to
> > > get drivers working originally, but it breaks the assumption
> > > in the asm-generic code.
> >
> > Yes, that is right.
> >
> > There is a number of common hardware modules that Freescale have
> > used in the ColdFire SoC parts and in their ARM based parts (iMX
> > families). The ARM parts are pretty much always little endian, and
> > the ColdFire is always big endian. The hardware registers in those
> > hardware blocks are always accessed in native endian of the processor.
> 
> In later Freescale/NXP ARM SoCs (i.MX and Layerscape), we
> also get a lot of devices pulled over from PowerPC, with random
> endianess. In some cases, the same device that had big-endian
> registers originally ends up in two different ARM products and one of
> them uses big-endian while the other one uses little-endian registers.
> 

Yes, this seems confirmed also from the drivers/dma/fsl-edma-common.h
comment:

/*
 * R/W functions for big- or little-endian registers:
 * The eDMA controller's endian is independent of the CPU core's endian.
 * For the big-endian IP module, the offset for 8-bit or 16-bit registers
 * should also be swapped opposite to that in little-endian IP.
 */

> > So the address range checks are to deal with those internal
> > hardware blocks (i2c, spi, dma, etc), since we know those are
> > at fixed addresses. That leaves the usual endian swapping in place for
> > other general (ie external) devices (PCI devices, network chips, etc).
> 
> Is there a complete list of coldfire on-chip device drivers?
> 

I can list those i worked on

i2c-imx.c
spi-fsl-dspi.c
mcf-edma.c + fsl-edma.common.c
now working on a sdhci-esdhc-mcf.c

And about mcf5441x, some other drivers as usb or probably can have still 
to be enabled/mainlined.  

> Looking at some of the drivers:
> 
> - drivers/i2c/busses/i2c-imx.c uses only 8-bit accesses and works either way,
>   same for drivers/tty/serial/mcf.c
> - drivers/spi/spi-coldfire-qspi.c is apparently coldfire-only and could use
>   ioread32be for a portable to do big-endian register access.
> - edma-common has a wrapper to support both big-endian and little-endian
>   configurations in the same kernel image, but the mcf interrupt handler
>   is hardcoded to the (normally) little-endian ioread32 function.
> - drivers/net/ethernet/freescale/fec_main.c is shared between coldfire
>   and i.MX (but not mpc52xx), and is hardcoded to readl/writel, and
>   would need the same trick as edma to make it portable.
> 
>       Arnd

Regards,
Angelo